The present invention relates to an improved method and device for producing plastic feed material containing long, parallel and aligned fibers for use in injection molding. More specifically, this invention relates to a die insert for an extrusion machine that inserts continuous fibers directly into the polymer feed material.
In the heat sink industries, it has been well known to employ metallic materials for thermal conductivity applications, such as heat dissipation for cooling semiconductor device packages. For these applications, such as heat sinks, the metallic material typically is tooled or machined from bulk metals into the desired configuration. However, such metallic conductive articles are typically very heavy, costly to machine and are susceptible to corrosion. Further, the geometries of machined metallic heat dissipating articles are very limited to the inherent limitations associated with the machining or tooling process. As a result, the requirement of use of metallic materials which are machined into the desired form, place severe limitations on heat sink design particular when it is known that certain geometries, simply by virtue of their design, realize better efficiency but are not attainable due to the limitations in machining metallic articles.
It is widely known in the prior art that improving the overall geometry of a heat-dissipating article can greatly enhance the overall performance of the article even if the material is the same. Therefore, the need for improved heat sink geometries necessitated an alternative to the machining of bulk metallic materials. To meet this need, attempts have been made in the prior art to provide molded compositions that include conductive filler material therein to provide the necessary thermal conductivity. The ability to mold a conductive composite enabled the design of more complex part geometries to realize improved performance of the part.
The attempts in the prior art included the employment of a polymer base matrix loaded with a granular material, such as boron nitride grains. Also, attempts have been made to provide a polymer base matrix loaded with long fibrous filler materials. These attempts are, indeed, moldable into complex geometries but still do not approach the desired performance levels found in metallic machined parts. In addition, known conductive plastic materials are undesirable because they are typically very expensive to manufacture because they employ very expensive filler materials. Still further, these conductive composite materials must be molded with extreme precision due to concerns of filler alignment during the molding process. Even with precision molding and design, inherent problems of fluid turbulence, collisions with the mold due to complex product geometries make it impossible to position the filler ideally thus causing the composition to perform far less than desirable.
Moreover, the typical injection molding process employs a pelletized thermosetting polymer feed stock. This creates further complication in the use of long fibrous fillers for several reasons. If the fibers are incorporated into the polymer at the time of injection molding the part by mixing the fibers into the base polymer during the melting process many of the fibers are broken by the turbulence of the mixing process. Further, if preformed pellet feed stock containing fiber filler is used the length of fibers contained therein are often shorter than the entire length of the pellet material and have an unpredictable overall length distribution. This is typically the result because the pellets are formed using the method described above where random length filler fibers are added to the base matrix material and mixed by a destructive screw or auger and then injection molded into a strand that is pelletized providing a fiber distribution throughout the feed pellet of random lengths with virtually all of the fibers being shorter than the overall length of the pellet.
Another process used for of adding continuous, parallel and aligned fiber reinforcing to the center of a plastic product involves pulling the fiber over several directional rollers, through some form of resin bath containing a molten polymer to fully wet the fibers and subsequently through a heating process and a final forming die. This method of feeding the fibers, however, requires multiple steps employing large equipment and is difficult to use when the fibers to be incorporated are brittle and susceptible to frequent breakage thus causing a great deal of machine down time and interruptions in the continuity of the fiber within the product. Although many types of reinforcing fiber can withstand this process and be incorporated into a final product that satisfies the final desired result of a fiber reinforced product, the type of fiber that must be incorporated in to the plastic in the field of thermally conductive plastics is very application specific and tends to be brittle.
In view of the foregoing, there is a demand for a composite material that is reinforced with continuous fibrous filler. In addition, there is a demand for a method of producing a composite feed stock material that contains continuous fiber reinforcing that can be further molded or cast into complex product geometries. There is also a demand for a pelletized injection molding feed stock material and a method of manufacturing that material that exhibits thermal conductivity as close as possible to purely metallic conductive materials while being relatively low in cost to manufacture.
In this regard, the present invention provides for a new die insert for an extrusion machine that allows a continuous strand of fiber reinforcing material to be statically drawn directly to the center of the molten flow of polymer base matrix. The insert is placed in line with the discharge end of an injection molding apparatus. The pressure injected molten flow of polymer base matrix proceeds in a straight linear fashion from the input end to the output end of the die. A fiber feed tube is placed in the die insert having an input end on the exterior of the die and an outlet end in the center of the polymer product flow.
As the base polymer flows past the outlet end of the feed tube the molten polymer comes into contact with the reinforcing fiber and fully wets out the fiber. As the molten polymer flows further down the die insert it begins to cool and become more viscous causing the polymer flow to begin to pull on the fiber reinforcing located at its center. This feeding action, caused by the cooling polymer, provides two advantages. First, it allows the fiber material to be placed into the polymer without having to be subjected to the pulling forces and multiple directional changes required under the current technology. Also, the fiber feed action is generated further downstream in a cooling section of the injection molding die, as the polymer begins to harden, thus pulling on the fiber strand (now polymer reinforced) and drawing new, brittle and unreinforced fibers into the still molten injection fed polymer material. In the field of thermally conductive plastic, this is an important feature in that it allows brittle fibers such as carbon fiber to be incorporated into the feedstock in relatively long intact continuous lengths. The result provides a continuous extrusion process that allows the formation of a continuous fiber-reinforced product employing brittle fibers in one-step process.
The material thus produced is then processed further by cutting the material with a pelletizing machine. The length at which the pellets are cut can be tailored to provide reinforcing fibers of the desired length. The pellets thus produced are of a high quality as required for use as feedstock in the injection molding of thermally conductive polymer parts where there is a need for high aspect ratio thermally conductive filler materials such a carbon fiber. Alternatively, the reinforced polymer may be injected directly into a mold cavity for forming a part.
Accordingly, among the objects of the instant invention is a device for producing pelletized injection molding feedstock having continuous reinforcing fibers therein. Another object of the present invention the provision of a low cost method for producing injection molding pellets having continuous thermally conductive fibers extending along the entire length of each of the pellets. Another object of the present invention is a device for producing a fiber reinforced polymer structure where the fiber is incorporated into the flow of polymer material using the static force of the polymer flow.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.